1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of thin film transistors (hereinafter, referred to as “TFTs”), and a method of manufacturing the same. In particular, the present invention relates to electronic equipment provided with a liquid crystal module, an EL module, and the like as its parts.
In the present specification, a semiconductor device refers to all the devices that function by using semiconductor characteristics, and includes an electro optical device, a light-emitting device, a semiconductor circuit, and electronic equipment.
2. Description of the Related Art
In recent years, semiconductor devices have been developed, which include a large integrated circuit formed of thin film transistors (TFTs), each TFT being formed of semiconductor films (thickness: about several nm to about several hundreds of nm) provided over a substrate having an insulating surface. Representative examples of the semiconductor device include an active matrix type liquid crystal module, an EL module, and a contact type image sensor. In particular, a TFT (hereinafter, referred to as a “polysilicon TFT”) utilizing a crystalline silicon film (typically, a polysilicon film) has high electric field effect mobility, so that it is capable of forming a circuit with various functions.
For example, in a liquid crystal module provided in a liquid crystal display apparatus, on the functional block basis a pixel circuit for conducting a pixel display, and a driving circuit for controlling pixel circuits such as a shift register circuit, a level shifter circuit, a buffer circuit, and a sampling circuit based on a CMOS circuit are formed on one substrate.
Furthermore, in a pixel circuit of an active matrix type liquid crystal module, a TFT (pixel TFT) is disposed for each of several tens to several millions of pixels, respectively, and a pixel electrode is provided for each pixel TFT. A counter electrode is provided on a substrate opposite to the pixel electrode with liquid crystal therebetween. Thus, a kind of capacitor using liquid crystal as a dielectric is formed. A voltage applied to each pixel is controlled by a switching function of a TFT, and liquid crystal is driven by controlling a charge to the capacitor, whereby the amount of transmitted light is controlled to display an image.
A pixel TFT is made of an n-channel TFT, and functions as a switching element to drive liquid crystal by applying a voltage. Liquid crystal is driven with an A.C. voltage, so that a system called frame inversion driving is mostly adopted. According to this system, in order to suppress power consumption, a pixel TFT is required to have characteristics of keeping an OFF current value (drain current flowing during an off operation of a TFT) sufficiently low.
As a structure of a TFT for reducing an OFF current value, a lightly doped drain (LDD) structure is known. In this structure, a region (called an LDD region) with low-concentration impurity elements added thereto is provided between a channel forming region and a source region/drain region formed by adding high-concentration impurity elements thereto.
In the case where a typical LDD region is formed in a conventional pixel TFT, an OFF current value can be reduced. However, an ON current value is also decreased. Although a typical LDD structure can suppress an OFF current value, the ability of the LDD structure of alleviating an electric field in the vicinity of a drain to prevent degradation due to hot carrier injection is low.
Furthermore, as means for preventing a decrease in an ON current value due to hot carriers, a so-called GOLD (gate-drain overlapped LDD) structure is known in which an LDD region and a gate electrode are overlapped with each other via a gate insulating film. In such a structure, an electric field intensity in the vicinity of a drain is alleviated to prevent injection of hot carriers, which is effective for preventing a degradation phenomenon.
Furthermore, a GOLD structure has a high ability of preventing a decrease in an ON current value, whereas it has a high OFF current value compared with a typical LDD structure. Thus, the GOLD structure is not preferable for being applied to a typical TFT.
In a semiconductor device having a plurality of integrated circuits such as an active matrix type liquid crystal display apparatus, the above-mentioned problems become conspicuous particularly in a crystalline silicon TFT, and they become more conspicuous as the performance required for an active matrix type liquid crystal display apparatus is enhanced.